1. Field of the Invention
The present invention relates to a device for the unambiguous measurement of the roll of a projectile and its application to the correction of he path of the projectile. It can be applied especially to the correction of the paths of projectiles fired by a gun and when the correction requires knowledge of the roll position of the projectiles.
To guide a projectile or a missile in flight, it is necessary to act judiciously on its driving devices such as the rudders, impellers or gas jets. If we consider for example the action of a side impeller incorporated into the projectile and designed to correct its path, it will be necessary to control the impellers at the right time, namely when the situation of the impeller is such that its action will guide the projectile in the right direction. Since the position of the impeller is perfectly defined with respect to the projectile, it is enough to know the roll position of this projectile. In other words, it is enough to know the orientation of the projectile with respect to the plane passing through a horizontal or vertical axis of the projectile and its longitudinal axis, namely the roll angle of this projectile.
2. Description of the Prior Art
There are prior art approaches used to find out the roll of a projectile either at the time of blast-off or during the flight phase. A first approach for measuring the roll of a projectile consists especially in acquiring this information when the projectile leaves the mouth of the gun. The projectile contains a magnetic component whose magnetic field vector is known. At the exit from the gun, two windings are positioned in such a way that a voltage is induced when the projectile leaves the gun. A computation unit uses this induced voltage to know the roll at the starting point of the flight phase. This approach is described especially in the patent application EP 0451122. The use of such a device nevertheless assumes precise knowledge of the law of variation of the rotational speed in roll during the flight phase. It is not generally possible to have this knowledge.
Another approach, described for example in the U.S. Pat. No. 5,039,029 consists in observing the characteristics of one or more signals sent out by the projectile. By transmitting a linearly polarized signal to the ground, the plane of transmission with respect to the geometry of the missile can be identified. This is possible at the ground by comparing the levels received in two orthogonal polarizations. However, a 180-degree ambiguity remains. To resolve this ambiguity, the projectile has an antenna array that may be driven in phase so as to generate a roll modulation. Signals of different levels are obtained for two directions spaced out by 180xc2x0 in the previously identified plane. This approach however has the drawback of requiring active components on board the projectile. The result thereof is a problem of cost as well as a problem of reliability because these components are not generally in a position to withstand the levels of acceleration imposed on the projectile without suffering damage.
The above approaches either do not measure the roll during the flight phase or require the incorporation of electronic components into the projectile. For reasons of reliability and cost in particular, it is preferable to implement an approach that does not bring active elements into play in the projectile and at the same time enables the measurement of the roll of the projectile. A passive approach of this kind can be devised on the basis of a simple radar observation.
Conventionally, a radar illuminates the projectile according to a given polarization, the signal back-scattered by the projectile being analyzed according to the same polarization as that used at transmission. Thus used, the radar gives only the position of the projectile. When the projectile is in rotation, no information about its roll position can be accessed since it substantially has a symmetry of revolution. Furthermore, this geometry of revolution generates no depolarization which it might be sought to exploit in order to acquire information relating for example to the sighting line pertaining to the axis of the projectile. For the projectile to have a signature dependent on its roll, it is necessary to modify the geometry thereof in such a way that, when illuminated by a radar wave, the amplitude of the back-scattered field depends on its roll. A geometrical solution may consist in incorporating appropriately sized grooves embedded in the rear casing of the projectile. These grooves constitute elements for which the back-scattered field depends highly on the incident polarization, the polarization of the radar being fixed. The dependence on the incident polarization results from the orientation of the grooves which are themselves in rotation because they form part of the projectile. This approach has the advantage of not requiring any active components on the projectile. However, it has a drawback of giving a measurement of the roll angle only to nearest value of xcfx80. There is therefore an 180xc2x0 ambiguity of measurement. Indeed, a groove is parallel to the direction of polarization of the electromagnetic field sent by the radar for an angle of roll of the projectile to the nearest value of xcfx80.
One aim of the invention especially is to overcome this drawback and therefore obtain an unambiguous measurement of the angle of roll of a projectile without having active components integrated into this projectile.
To this end, an object of the invention is a device for the measurement of the angle of roll of a projectile, comprising at least:
a radar equipped with means of processing and sending a signal to the casing of the projectile in at least one direction of incident polarization {overscore (E)};
a set of parallel grooves made on the casing, the depth of which is modulated dissymmetrically with respect to the axis of symmetry of the projectile;
the axis of symmetry of the projectile not passing through the point of the antenna of the radar where the antenna beam is generated, the processing means analyzing, in reception, a signal back-scattered by the casing of the projectile, the signal being modulated as a function of the angle of roll of the projectile, the modulation having two maximum local values corresponding to two angular roll positions of the projectile such that the polarization {overscore (E)} is parallel to the grooves, the processing means removing the 180xc2x0 ambiguity by comparing the levels of the local maximum values.
In a particular embodiment, the depth of the grooves goes on increasing.
The main advantages of the invention are that it increases the reliability of measurement of the roll and is simple to implement and economical.